A) Because the Precursors didn't fight back. They simply observed the destruction the Forerunners were capable of. And their dominion stretches across the universe. Forerunners didn't come close to destroying them all. What did, however is... B) The way they tried to store themselves for extended periods of time. It was the Flood. Unfortunately, the Flood, well, did what Flood does, and pretty much deformed every one of them.